Blight
From Arkham Horror Wiki
Blight cards represent important people who live in the town of Arkham and who have been driven mad by seeing "The King in Yellow." One Blight card enters play each time a Yellow Sign token is placed on the terror track. Once they enter play, Blight cards have a global effect on the game and cannot be gotten rid of. For example, once Oliver Thomas enters plays, the cost to seal gates is increased by 1 Clue token for the rest of the game. In addition, if you draw an Arkham Encounter card that mentions the character in its text, you ignore that card and lose either 1 Stamina or 1 Sanity due to the encounter with the now insane resident.
